Flat-panel displays are widely used in computing devices, in portable devices, and for entertainment devices such as televisions. Such displays typically employ a plurality of pixels distributed in an array over a display substrate to display images, graphics, or text. For example, liquid-crystal displays (LCDs) employ liquid crystals to block or transmit light from a backlight behind the liquid crystals. Organic light-emitting diode (OLED) displays rely on passing current through a layer of organic material that glows in response to the electrical current. Each pixel usually includes three or more sub-pixels emitting light of different colors, for example red, green, and blue.
Displays are typically controlled with either a passive-matrix (PM) control employing electronic circuitry external to the display substrate or an active-matrix (AM) control employing electronic circuitry formed directly on the display substrate and storing data associated with each light-emitting element. Both OLED displays and LCDs using passive-matrix control and active-matrix control are available. An example of such an AM OLED display device is disclosed in U.S. Pat. No. 5,550,066.
Typically, in an active-matrix-controlled display each display sub-pixel is controlled by one control element, and each control element includes at least one transistor. For example, in a simple active-matrix OLED display, each control element includes two transistors (a select transistor and a drive transistor) and one capacitor for storing a charge specifying the desired luminance of the sub-pixel. Each OLED element employs an independent control electrode connected to the power transistor and a common electrode. In contrast, an LCD typically uses a single-transistor circuit. Control of the light-emitting elements is usually provided through a data signal line, a select signal line, a power connection and a ground connection. Active-matrix elements are not necessarily limited to displays and can be distributed over a substrate and employed in other applications requiring spatially distributed control.
Active-matrix circuitry is commonly achieved by forming thin-film transistors (TFTs) in a semiconductor layer formed on a display substrate and employing a separate TFT circuit to control each light-emitting pixel in the display. The semiconductor layer is typically amorphous silicon or poly-crystalline silicon and is distributed over the entire flat-panel display substrate. The semiconductor layer is photolithographically processed to form electronic control elements, such as transistors and capacitors. Additional layers, such as insulating dielectric layers and conductive metal layers, are provided (e.g., often by evaporation or sputtering) and photolithographically patterned to form electrical interconnections, structures, or wires.
Surface-mount devices (SMDs) are an alternative way to provide electrical elements on a substrate or backplane. Such devices, as their name suggests, include electrical connections that are typically placed on the surface and in contact with a backplane rather than including that extend through vias in the backplane. Surface-mount technology (SMT) is widely used in the electronics industry to provide high-density printed-circuit boards (PCBs). In particular, a well-developed and inexpensive infrastructure exists for making and integrating two-terminal surface-mount devices, such as resistors or capacitors, into printed circuit boards. However, the smallest surface-mount device readily available is several hundred microns long and wide, precluding their use for applications requiring integrated circuits with circuit elements having a size of several microns or less.
There is a need, therefore, for devices and manufacturing methods that enable the use of two-terminal devices for complex circuits in electronic systems.